My grandson has been addicted to oxicodone. Can he still be having withdraws even weeks later? I think it is all mental not phy

I always thought that a person went through withdraws from oxicodone as soon as the drug wore off If a person is addicted to it. My grandson tells his Dad that he is having withdraws even weeks after he has been off the pills. My son believes him. I say it is all mental. Who is right?